This invention relates to nuclear reactors and more particularly to structure for maintaining the core in proper alignment during reactor operation.
A typical modern nuclear power reactor contains over two hundred elongated fuel assemblies supported in close side-by-side relationship within a nuclear reactor vessel. Th assemblies as a group are typically referred to as a nuclear reactor core, which produces heat as a result of the nuclear chain reaction. The reactor vessel has an inlet nozzle for introducing the cooling medium, typically water at a pressure of over 2000 psi, into the upstream end of the core. The water absorbs heat as it traverses the core and then exits the vessel to directly or indirectly drive a turbine.
In order to operate the reactor core safely, the power distribution must be carefully controlled to avoid local power peaking. Maintaining proper spacing between individual fuel assemblies is an important factor in maintaining a satisfactory power distribution. Safe operation also requires that the control rods, which are used to shut down the nuclear chain reaction, always have an uninterrupted path should they be inserted into or between the fuel assemblies. Any distortion in the shape of or separation between fuel assemblies could prevent proper insertion of the control rods. It may thus be appreciated that the shape of the nuclear reactor core and the spacing between individual assemblies must be maintained.
Despite careful design to prevent distortion of fuel assemblies during operation, a condition known as fuel assembly bowing has occasionally ben observed in operating reactors. Although all the factors contributing to bowing are not fully understood, it is believed that a combination of factors including the relatively low lateral stiffness of the fuel assemblies, the cross flows existing in the core, and the compressive forces on the assemblies provided to hold them down against the flow forces, give rise to the bowing problem. Bowing may occur on an individual fuel assembly, but core bowing has also been observed. Because of the spacing between the perimeter of the core and the closest rigid structure such as the core shroud, and the spacings between individual fuel assemblies, an imbalanced but steady force, such as an asymetry in the core flow, tends to distort all the assemblies in the same direction. This results in a substantial bowing of the core such that one side of the core is concave and the other is convex.